The present invention relates to new precursor compounds useful for the chemical vapor deposition (CVD) of materials which are binary combinations of group 13 elements (commonly known as group 3 elements) (Al, Ga, In) and group 15 elements (commonly known as group 5 elements) (P, As, Sb).
Organometallic compounds have been used to prepare thin films of a wide variety of materials by chemical vapor deposition. The production of compound semiconductors, metals and dielectrics has been of considerable interest. It has been known that metals and inorganic compounds can be prepared using organometallic sources, but it was felt that the materials would suffer severely from contamination from e.g. carbon, silicon and oxygen. It was not until the late 1960s that the approach of chemical vapor deposition was taken seriously. The technique known as metal-organic chemical vapor deposition (MOCVD) has since been investigated in many research laboratories throughout the world and has emerged as a powerful method for the preparation of thin films of electronic materials. A major part of the effort has concentrated on the investigation of III-V semiconductor layers, with GaAs and Ga.sub.1-x Al.sub.x As (wherein X is from 0 to 1) alloys receiving particular attention because of their commercial importance for high speed, microwave, and opto-electronic device applications. Since these materials have been studied most extensively, and because of their importance, they may be used as examples in the description of the MOCVD technique. Although the technique has not been developed to such an advanced stage for other materials, rapid progress is being made, for example, in the deposition of indium phosphide-based III-V alloys such as (GaIn)As, (AIIn)As, and (GaIn)(AsP), II-VI semiconductors, metals, and oxides.
Examples of typical reactions previously employed in MOCVD to prepare films of gallium arsenide, zinc selenide, aluminum, and tin oxide are as follows: ##STR1##
FIG. 1 schematically shows a system usable for preparation of GaAs/Ga.sub.1-x Al.sub.x As (wherein x is from 0 to 1) epitaxial layers by MOCVD. In this particular schematic example, trimethyl gallium, trimethyl aluminum, and arsine have been used to prepare gallium arsenide or gallium aluminum arsenide, with hydrogen selenide and diethyl zinc serving to dope the materials n-type or p-type respectively. The prior organometallic precursors are liquids (or, less frequently, solids) and can be transported to a reactor by bubbling a carrier gas through the liquid which is held at an accurately controlled temperature to ensure a constant vapor pressure. By precisely metering the flow of the carrier gas, hydrogen, using a mass flow controller, the concentration of the organometallic precursor in the vapor phase can be reliably and reproducibly controlled. Electronic grade organometallic precursors are usually supplied commercially in bubblers manufactured in stainless steel, whereas the hydrides are available as gas mixtures in hydrogen. The reagents generally enter the quartz reactor in the gas phase and flow over, for example, a single crystal GaAs or silicon substrate which is situated on a temperature controlled graphite susceptor heated by radio frequency induction. Deposition can be carried out at atmospheric or reduced pressure (e.g. 0.1 Atm).
Epitaxial growth of the semiconductor layer occurs at growth rates which usually lie in the range 500.ANG.-1000.ANG./min, and layer thicknesses in the range 10.ANG.-100.mu.m can be achieved. Materials can be doped n-type or p-type in the range 10.sup.15 -10.sup.19 cm.sup.-3. In modern systems the reagents are introduced using a gas manifold which allows rapid switching of gas flows so that abrupt changes in alloy composition or doping can be achieved. The sequencing of events is controlled by computer and this allows the fabrication of complex heterostructures. The versatility of the technique makes it attractive for the preparation of epitaxial material for conventional device structures as well as the more advanced low dimensional solids such as quantum wells and superlattices.
MOCVD has been used to prepare virtually all of the possible III-V binary compounds (Al, Ga, In/N, P, As, Sb and a variety of ternary and quaternary materials such as Ga.sub.1-x In.sub.x As and Ga.sub.1-x In.sub.x As.sub.1-y Sb.sub.y (wherein x and y are each 0 to 1) compounds and alloys of the III-V type prepared by MOCVD are as follows:
______________________________________ Binary compounds AlN AlP AlAs AlSb GaN GaP GaAs GaSb InP InAs InSb Ternary compounds Ga.sub.(1-x) Al.sub.x As Ga.sub.(1-x) In.sub.x As Al.sub.(1-x) In.sub.x As Ga.sub.(1-x) In.sub.x P GaAs.sub.(1-y) P.sub.y GaAs.sub.(1-y) Sb.sub.y InAs.sub.(1-y) P.sub.y Quaternary compounds Ga.sub.1-x In.sub.x As.sub.1-y P.sub.y Ga.sub.1-x In.sub.x As.sub.1-y Sb.sub.y ______________________________________
Exemplary reagents which have been used in such preparation of III-V compounds and alloys are as follows:
______________________________________ Metal alkyls Ga(CH.sub.3).sub.3, Ga(C.sub.2 H.sub.5).sub.3 Al.sub.2 (CH.sub.3).sub.6, Al(C.sub.2 H.sub.5).sub.3 In(CH.sub.3).sub.3, In(C.sub.2 H.sub.5).sub.3 Hydrides & Group V alkyls NH.sub.3, AsH.sub.3, PH.sub.3, As(CH.sub.3).sub.3, P(CH.sub.3).sub.3, Sb(CH.sub.3).sub.3 Adducts (CH.sub.3).sub.3 In: P(CH.sub.3).sub.3 (CH.sub.3).sub.3 In: P(C.sub.2 H.sub.5).sub.3 (CH.sub.3).sub.3 In: N(CH.sub.3).sub.3 (CH.sub.3).sub.3 Ga: P(C.sub.2 H.sub.5).sub.3 (CH.sub.3).sub.3 Ga: As(CH.sub.3).sub.3 Dopants p-type Zn(CH.sub.3).sub.2, Zn(C.sub.2 H.sub.5).sub.2, Cd(CH.sub.3).sub.2, Mg(C.sub.5 H.sub.5).sub.2, Be(C.sub.2 H.sub.5).sub.2 n-type SiH.sub.4, Si.sub.2 H.sub.6, Sn(C.sub.2 H.sub.5).sub.4, GeH.sub.4, Sn(CH.sub.3).sub.4 , H.sub.2 S, H.sub.2 Se, Te(C.sub.2 H.sub.5).sub.2 Semi-insulating Cr(CO).sub.6, VO(OC.sub.2 H.sub.5).sub.3, Fe(C.sub.5 H.sub.5).sub.2 ______________________________________
It is extremely important that the reagents are free from contaminants which, if they become incorporated in a semiconductor layer, can have an adverse influence on the electrical and optical properties of the material. Purity levels well below 1 ppm have to be achieved if background carrier concentrations in the semiconductors are to be 10.sup.14 cm.sup.-3 or less, and methods of improving the purity of the precursors are being intensively investigated. Unfortunately, methods of analysis of impurities at these levels are proving to be somewhat difficult, they are generally assessed by study of the electrical properties of the completed film.
Listed below are the properties required of an organometallic precursor under ideal circumstances:
Its vapor pressure should be greater than 10mm at room temperature or below. This is not essential since many of the adducts used for III-V deposition have lower vapor pressures. However, providing the compound is thermally stable it is simpler in practice not to have to heat all the gas lines and valves in the gas handling system to prevent condensation of reagents from the gas phase PA1 It is preferable that the organometallic compound be liquid at the temperature used. There is evidence to suggest that when a solid reagent is used the pick-up is variable and this is possibly caused by the changing surface area of the solid as it is consumed. PA1 The impurity content of the precursor should be well below a level of 1 ppm. At the moment, the simplest way of testing the quality of a reagent is to prepare a semi-conductor layer and assess its electrical and optical properties. A great deal of time could be saved if a high sensitivity analytical technique could provide fast characterization of precursors PA1 Non-pyrophoric compounds would be preferable, so that fire hazards could be minimized. The majority of precursors in current use are pyrophoric, and it seems likely that this will continue to be the situation PA1 Reagents have to be stable in their containers over a period of years since their rate of consumption is rather low (100 gms is a typical quantity contained in a bubbler). If vapor phase concentrations of reagents are to remain reproducible from day to day and month to month, chemical changes within the containers are unacceptable PA1 The container in which the precursors are delivered must be designed in such a way that they can be dispensed simply and without hazard.
Metal-organic chemical vapor deposition has emerged as a powerful technique for the preparation of materials for III-V and II-VI electronic devices, and the research currently being undertaken on the preparation of oxides and metals by thermal and photolytic processes is likely to extend the range of applications of materials prepared using the technique. As the range expands there will be an increasing need for a wider variety of electronic grade organometallic precursors. The present invention involves w improved precursors and corresponding reduced and controlled impurity levels in electronic grade organometallic reagents.
Despite their potential importance as precursors to semiconductors such as gallium arsenide and indium phosphide, relatively little is known about compounds featuring bonding between the heavier group 13 and 15 (Olander numbering) elements. (Tuck in Comprehensive Organometallic Chemistry, eds. Wilkinson, et al. Pergamon Press, New York, (1982) vol. 1. ch. 7. pp. 683-723). Pioneering work by Coates et al. revealed that secondary phosphines and arsines undergo thermal reactions with Me.sub.3 Ga or Me.sub.3 In to afford materials of composition (Me.sub.2 MER.sub.2)n, (when R=methyl (Me) n=3 and when R=phenyl [Ph], n =3; M =Ga or In; E =P or As). (Coates et al., J. Chem. Soc., (1963) 233; Beachley et al., J. Chem. Soc., (1965) 3241). Until recently, no structural information was available for compounds featuring direct sigma bonding (as opposed to dative bonding) between the heavier group 13 and group 15 elements.
Wells, et al., have reported the X-ray structure of the dimer [(Me.sub.3 SiCH.sub.2).sub.2 AsGaPh.sub.2 ].sub.2 which is obtained via the reaction of (Me.sub.3 SiCH.sub.2).sub.2 AsH with Ph.sub.3 Ga (J. Organometallic Chem. (1986), 308, 281). Beachley, et al., have reported the structure of the dimeric indium phosphide complex [(Me.sub.3 SiCH.sub.2 InPPh.sub.2 ].sub.2 (J. Organometallic Chem. (1987), 325, 69).
Other structural information comes from a gas phase electron diffraction study of the trimeric aluminum phosphide [Me.sub.2 AlPMe.sub.2 ].sub.3 (Haaland, et al., J. Organometallic Chem. (1987) 322, C24). More recently, Wells, et al., have reported the novel gallium-arsenic cluster. [(PhAsH)(R.sub.2 Ga)(PhAs).sub.6 (RGa).sub.4 ] (R=Me.sub.3 SiCH2) (Wells, et al. J. Chem. Soc. Chem. Commun. (1986), 487.
The structures of the monomeric and dimeric tris(arsino) gallanes [(Mesityl).sub.2 As].sub.3 Ga and {[(Me.sub.3 SiCH.sub.2).sub.2 As].sub.3 Ga}.sub.2 have also recently been reported (Wells, et al., Inorg. Chem. (1986), 25, 2484, and J. Organometallic Chem. (1987), 325, C7).
The synthesis of [[(Me.sub.3 SiCH.sub.2).sub.2 As].sub.2 GaCl]2 has recently been reported (Pitt, et al. Organometallics, (1986), 5, 1266). The trinuclear derivatives [Et.sub.2 M-PEt.sub.2 ] (M=Ga, In) and [Cl,MeGa-PEt.sub.2 ].sub.n (n=2.6) have also been described (Maury, et al., Polyhedron, (1984), 3, 581).